Numerous state of the art plants for the continuous casting of thick slabs and blooms are known. These plants use containment and guiding rollers arranged under the ingot mould to support and accompany along the bend below the product exiting the ingot mould whilst the solidification of all the steel takes place. On exiting the ingot mould the cast product is only partially solidified and is constituted by the solidified external part, known as the skin or shell, with a thickness that grows as the distance from ingot mould outlet increases, and a still liquid part, the so-called core or metal core. The solidification endpoint, i.e. the tip of the metal core, may be more or less distant from the ingot mould outlet section according to the casting speed of the plant. As continuous attempts are made to increase the casting speed in order to increase casting plant productivity, the length of the metal core may sometimes be important.
Due to the presence of liquid steel inside the cast product exited from the ingot mould, ferrostatic pressure exists, which presses from inside towards the exterior and causes the skin to deform in the stretches in which it is not supported by the containment and driving rollers: due to this ferrostatic pressure, bulges form on the surface of the cast product in the areas comprised alternatively between two rows of adjacent rollers. This phenomenon is known as “static bulging”.
One problem that originates from the presence of such bulges is that of the surging of the liquid steel upwards due to the buckling of the bulges produced by the rollers during the advance of the thick slab along the rollerway. This pulsation is transmitted to the entire liquid metal column causing a pulsed variation of the meniscus level inside the ingot mould. This phenomenon is known as “dynamic bulging” and is amplified by the increase in casting speed; the meniscus oscillations may be so high as to make control of the ingot mould levels impossible with a consequent necessary interruption of the casting process and production loss. In the worst cases, meniscus oscillations may be such as to cause the liquid steel to leak from the ingot mould.
Another drawback caused by dynamic bulging is that relating to the internal and superficial skin cracks, which form due to the periodical bulging of the skin; these cracks create the presence of defects in the end product and may favour the rupture of the skin with consequent leakage of the liquid steel, a phenomenon known as “breakout”.
A casting plant that offers a solution to the drawback is disclosed by U.S. Pat. No. 6,308,769, which describes a casting plant in which a thick slab guiding device is envisaged arranged immediately at the outlet from the ingot mould. The device is constituted by a number of segments that support series of rollers that constitute the thick slab containment guides. The rollers on one side are arranged with their axes staggered with respect to the opposite side of the thick slab; and the pitch of the rollers on both sides always remains constant. This particular arrangement is maintained for a stretch in which a liquid core is present in the thick slab. In any case, the solution offered by this plan is not satisfactory in those cases in which the casting speed is excessively high. In fact, the pulsation due to the passage of the bulges between the rollers continues to occur and is also due to the large volume of the bulges that in any case occur between two adjacent rollers. FIG. 1A, which illustrates a front view of a known casting plant of the type described in U.S. Pat. No. 6,308,769, shows the amplitude of the area interested by the bulging phenomenon between two rollers, distanced at a constant pitch “p”, and indicated with the dotted line.
Furthermore, a staggered arrangement of the rollers as envisaged by the above mentioned document does not make it possible to perform “soft-reduction”, in order to reduce the thickness of the thick slab on outlet from the ingot mould and to improve the internal quality thereof.
A plant for the continuous casting of metal products that overcomes these drawbacks is therefore required.